One of the important physiological systems in the human body is the cardiovascular system, which includes the heart, the arterial system and the venous system. Many imaging modalities can be used to investigate the structural properties of the cardio-vascular system, including, angiography, X-ray CT, MRI, SPECT and ultrasound. However, it is difficult to non-invasivly determine haemodynamic properties of the cardiovascular system. Determining the following haemodynamic properties is particularly useful:
(a) blood flow volume in a particular arterial/venous subsystem; PA1 (b) local turbulence and pressure drops, which may indicate stenoses and/or aneurysms; PA1 (c) pulse wave velocity; PA1 (d) cardiac left ventricular stroke volume; and PA1 (e) the reaction of arteries and veins to a pulse wave. PA1 (a) blood velocity; PA1 (b) blood flow rate; PA1 (c) pulse wave velocity; PA1 (d) vessel dilation; PA1 (e) dynamic vessel radius and/or cross-section; PA1 (f) turbulence; PA1 (g) estimate of stenosis percentage; and PA1 (h) blood pressure. PA1 irradiating the conduit with ultrasonic waves; detecting first Doppler-shifted reflections of the waves from the material, which first reflections have a positive Doppler-shift; detecting second Doppler shifted reflections of the waves from the material, which second reflections have a negative Doppler-shift; and estimating the flow velocity based on the extent of the positive and negative Doppler-shifts. PA1 (a) irradiating the conduit with first ultrasonic waves from a first direction such that the first waves have a component of propagation in the flow direction of the material; and PA1 (b) irradiating the conduit with second waves from a second direction such that the second waves have a component of propagation opposite the flow direction of the material, PA1 where at least a portion of first waves are reflected as the second reflections and at least a portion of the second waves are reflected as the first reflections. Preferably, the first waves and the second waves intersect. More preferably, they intersect at the conduit. Alternatively, the waves do not intersect. PA1 (a) detecting first reflections of waves from the first location; PA1 (b) detecting second reflections of waves from the second location; and PA1 (c) detecting third reflections of waves from the third location. PA1 non-invasivly determining a time-dependent blood pressure at a first location on the body; and PA1 repeating the blood pressure determination at a plurality of locations. PA1 a first ultrasonic transducer which transmits first ultrasonic waves towards the conduit at a first angle to the direction of the flow of the material and which receives first Doppler-shifted reflections of the first waves from the material; PA1 a second ultrasonic transducer, spaced from the first transducer, which transmits second ultrasonic waves, at a second angle to the first waves, towards the conduit and which receives second Doppler-shifted reflections of the second waves from the material; and PA1 a controller which estimates the first angle based on the distance between the first and second transducers, the second angle and the first and second reflections and which determines the velocity of the flow based on the determined first angle and the first reflections, where the propagation directions of the first and second waves have components in the same direction relative to the flow direction of the material. PA1 a first ultrasonic transducer which transmits first ultrasonic waves towards the conduit at a first angle to the direction of the flow of the material and which receives first Doppler-shifted reflections from the material; PA1 a second ultrasonic transducer, spaced from the first transducer, which transmits second ultrasonic waves, at a second angle to the first waves, towards the conduit and which receives second Doppler-shifted reflections from the material; and PA1 a controller which determines the velocity of the flow by summing the first and second reflections, where the propagation directions of the first and second waves have components which are parallel to the direction of the flow of the material and opposite from each other. Preferably, the controller estimates velocity without individually correcting velocity estimates derived from the first and second reflections for deviations related to the first angle. PA1 a first diameter-change detecting ultrasonic transducer which transmits third ultrasonic waves to the conduit and which receives first diameter-change indicating reflections of the third waves; and PA1 a second diameter-change detecting ultrasonic transducer which transmits fourth ultrasonic waves to the conduit and which receives second diameter-change indicating reflections of the fourth waves, where the controller determines a delay in the propagation of a particular pulse phase between the first and second locations from the diameter-change indicating reflections and delays a signal generated by the first Doppler-shifted reflections relative to a signal generated by the second Doppler-shifted reflections based on the determined delay. PA1 a first tube which conveys blood from a patient to an aerator; a second tube which conveys blood from the aerator to the patient; and a gas bubble detector, which detects gas bubbles in the blood in at least one of the first and second tubes. PA1 transmitting ultrasonic waves to the conduit at a first angle between the waves and the conduit, of between 10.degree.-70.degree. and having an uncertainty; receiving Doppler-shifted reflections from the material; and estimating the velocity based on the reflections to within an error, compared to the velocity, of less than the cosine of the uncertainty, without correcting for deviations caused by the uncertainty.
Doppler ultrasound imaging is widely used to determine local blood velocity flow in arteries. The frequency shift .DELTA.f of an ultrasonic wave of wavelength X, which impinges particles moving at a velocity v, is given by the following equation: ##EQU1##
In the case of blood, the ultrasonic wave is reflected mainly from red blood cells, which move in the same direction as the blood flow. It is relatively straightforward to transmit a wave of a known wavelength to a blood vessel, detect a reflected wave and determine the velocity of the blood based on equation (1). However, the magnitude of the frequency shift .DELTA.f is affected not only by the blood velocity v but also by an angle of incidence .alpha. and a reflectance angle .beta. of the wave from the blood. Typically in prior art measurements, these angles are not definitely determined, leading to an error of over 15% in velocity determination, as will be shown below with reference to FIG. 2.
One known method of more precisely determining the angle of incidence .alpha. and the reflectance angle .beta. is to generate an ultrasonic image including the blood vessel. The angles .alpha. and .beta. are measured on the image along an imaging beam which is used for Doppler frequency shift determination. However, due to limitations of the ultrasonic imaging modality, the angle determination is not sufficiently precise and a significant error in velocity determination can be expected. In addition, the price and complexity of an ultrasonic imaging system are much higher than that of a simple Doppler-ultrasound system.
There also exist many invasive methods for determining haemodynamic properties. One method includes inserting a sensor carrying catheter into a vein or artery. Haemodynamic properties, such as pressure or velocity, are very precisely sensed by the sensor and are converted into a host of dependent properties, such as blood vessel impedance.
Another, less invasive, method is magnetic flow determination. In this method, a coil is placed around an intact blood vessel. A magnetic field is induced in the coil. Since blood is a conductor, the flow of blood in the magnetic filed induces a voltage across the blood vessel which is proportional to the flow.
There is however a need for a precise, non-invasive method for determining blood velocity and other haemodynamic properties.
Blood velocity determination and detection of small gas bubbles in the blood are related, since encapsulated gas bubbles are readily detectable using Doppler-ultrasound. Gas bubbles do not naturally occur in the vascular system. Certain medical procedures, such as CPB (cardiopulmonary bypass), and certain types of physical trauma, may infuse air bubbles into the vascular system. In addition, certain activities, most notably sea-diving, can cause the creation of air bubbles in the vascular system. In general, gas bubbles cause severe and lasting neurological effects, as well as other grievous bodily damage. However, it is relatively difficult to monitor the formation of gas bubbles in vivo. As a result, sea-divers use non-personalized diving tables which estimate the danger from gas bubbles in the divers blood.
As mentioned above, Doppler-ultrasound can be used to detect gas bubbles, however, the current state of the art does not provide means sensitive enough to detect small, yet damaging, gas bubbles in the blood. In addition, there is no commercially available simple apparatus for monitoring of gas bubbles in sea-divers.
Gas bubbles can also be detected, counted and monitored under a microscope., however, this type of monitoring is generally not practical even for medical situations, as well as being extremely invasive.
Blood pressure is usually determined using a pressure cuff on the brachial artery in the arm. This method has a relatively low precision and only measures the systolic and diastolic pressures. In "Ultrasonic System for Noninvasive Measurement of Hemodynamic Parameters of Human Arterial-Vascular System", by Powalowski T., Archives Of Acoustics, Vol. 13, Issue 1-2, p. 89-108 (1988) and in "A Noninvasive Ultrasonic Method for Vascular Input Impedance Determination Applied in Diagnosis of the Carotid Arteries", by Powaiowski T., Archives Of Acoustics, Vol. 14, Issue 3-4, p. 293-312 (1989), the disclosures of which are incorporated herein by reference, a method of estimating instantaneous blood pressure in a blood vessel is described. However, Powalowski's estimation method has at least two limitations. First, estimating the instantaneous blood pressure requires determining the diastolic and systolic blood pressures using a pressure cuff Second, the method used for determining the velocity of the blood flow in the blood vessel is not very precise.
U.S. Pat. No. 5,488,953 to Vilkomerson, the disclosure of which is incorporated herein by reference, describes a method and a device for measuring the flow velocity in a blood vessel using ultrasonic Doppler processing, independent of an orientation of the device. However, the method described in the '953 patent requires that the angles between a plurality of ultrasonic beams and the flow be determined.
An article titled "Vector Doppler: Accurate Measurement of Blood Velocity in Two Dimensions," by John R. Overbeck. Kirk W. Beach and D. Eugene Strandness, Jr., Ultrasound in Medicine and Biology, Vol. 13, No. 1, pp. 19-31, printed by Pergamon press, USA, 1992, and which is incorporated herein by reference describes several multi-transducer Doppler ultrasound devices, in which the angle of incidence is determined by analyzing the difference in Doppler spectra of different transducers.
U.S. Pat. No. 5,406,854, the disclosure of which is incorporated herein by reference, describes a Doppler flowmeter in which signals from several receivers, all of which are similarly oriented to the flow, are mixed to increase the system sensitivity.
U.S. Pat. No. 5,119,821, the disclosure of which is incorporated herein by reference, describes an ultrasonic probe having two probes oriented at about 45 degrees to each other. By comparing the signals received by the two probes from a blood flow, constrictions in the flow can be determined.
Woodcock, J. P., "The Transcutaneous Ultrasonic Flow-Velocity Meter in the Study of Arterial Blood Velocity," Proceedings of the Conference on Ultrasonics in Biology and Medicine, UBIMED-70, Jablonna-Warsaw, Oct. 5-10, 1970, the disclosure of which is incorporated herein by reference, describes the use of two simultaneous measurements of flow parameters to ascertain the condition of corollary blood vessels.